1. Field of the Invention
The present invention relates to a base station for transmitting a retransmission packet when an error is detected in a transmission packet transmitted to a mobile station.
2. Description of the Related Art
Retransmission control schemes are used in radio communication systems, in which an error in a packet transmitted between a base station and a mobile station is detected and only a packet with the error is retransmitted. In view of retransmission timings, the retransmission control schemes are classified into two schemes as shown in FIG. 1. One scheme is referred to as Synchronous ARQ (automatic repeat request) and the other scheme is referred to as Asynchronous ARQ.
According to Synchronous ARQ, packets are retransmitted at predetermined retransmission timings. For example, when a transmission packet at a timing #0 (S00) is detected as an error packet, the corresponding retransmission packets can be transmitted at timings #0 (S10 and S20). Typically, retransmissions are performed at frames which are integral multiples of RTT (Round Trip Time) or control loop delay.
According to Asynchronous ARQ, packets are retransmitted at any timing after one RTT. For example, when a transmission packet at a timing #0 (A00) is detected as an error packet, the corresponding retransmission packets can be transmitted at any transmission slot timing (TTI: Transmission Time Interval) after A10. In other words, retransmissions are performed at any TTI (A10-A25) after one RTT from the transmission packet. Asynchronous ARQ is used in HSDPA (High Speed Downlink Packet Access) (see 3GPP TS 25.212, “Multiplexing and channel coding (FDD)”, for example).
On the other hand, Persistent Scheduling is used in radio communication systems, in which a base station allocates radio resources according to a predetermined cycle and a predetermined frequency allocation pattern. FIG. 2 shows an example of Persistent Scheduling. FIG. 2 specifically shows an allocation pattern of radio resources allocated to a mobile station (user data) by the base station. According to Persistent Scheduling, radio resources are allocated to user data according to a predetermined cycle and a predetermined frequency allocation pattern. For example, the base station allocates radio resources by means of a pattern which changes allocation every 5 TTIs in the time direction and every frequency block in the frequency direction. In the shown example, after the base station allocates a radio resource to a mobile station (user data) at a timing T0 and a frequency block F0, the base station allocates a radio resource to the mobile station at a timing T5 and a frequency block F1. The predetermined cycle and the predetermined frequency allocation pattern are known to both the base station and the mobile station. Thus, the mobile station can receive, demodulate, and decode data according to the predetermined cycle and the predetermined frequency allocation pattern. According to Persistent Scheduling, the base station need not transmit radio resource allocation information to the mobile station, since both the base station and the mobile station share the radio resource allocation information. As a result, Persistent Scheduling has an advantage of reduced overhead. For example, Persistent Scheduling can be applied to real-time voice communications in which radio resources should be allocated on a constant cycle (applications with the requirement of reduced overhead, in which communications are performed with a constant transmission speed and at a low rate).